A Very Brief History of the Universe:Notes:Giga (abbreviated G) is the international term for 1,000,000,000 or 109. In the U.S. this is called a billion – almost everywhere else it is a milliard. We use G to avoid confusion. M is for million. The universe is about 13G years old.
eV are electron-volts; and electron has a mass-energy (E=mc2, remember) of 0.511 MeV.
rad = radiation, CBR= cosmic background radiation, QSO=quasars, BH=Black hole, Msun= mass of our Sun.
A.U. is astronomical units (Sun-Earth distance, = 8 light-minutes); pc is parsecs: 2.36 light-years.

The observed expansion of the universe – the redshifts of galaxies first described by Edwin Hubble in 1929 implied a beginning to the universe, as had been suggested by earlier philosophers.

The observed abundances of deuterium, helium, and lithium; production of the observed quantities of hydrogen's heavier isotope and the next two heavier elements is thought to be due primarily to their synthesis in the first three minutes of the Big Bang. These elements are not produced in the required quantities in observed stellar fusion reactions.

The thermal spectrum of the Cosmic Microwave Background Radiation (CMBR) was predicted by Big Bang theory before its observation - always a convincing argument!

The CMBR appears hotter in distant clouds of gas. The speed of light is finite, so we are seeing these distant clouds at an earlier epoch, when the universe was denser and hotter, as expected from Big Bang Theory.

The Steady State alternative

The Steady State Model of scientists Hermann Bondi and Thomas Gold (and augmented by Fred Hoyle) postulated that there was no origin to the universe, that the large-scale features of the universe are constant from one epoch to the next, and thus to maintain the average density of galaxies in an expanding universe, whole new galaxies must be popping into existence between the previous ones. In addition, to explain the CMBR, a whole new class of 1014 weak microwave emitting sources must exist. This is “about 100,000 times the total number of visible galaxies” (according to Hoyle). More modern estimates place the number of galaxies at about 1011, or "only" 1,000 times fewer. This lack of supporting evidence for the Steady State theory and the perfect match between Big Bang predictions and the later discovery of the CMBR has led to almost universal acceptance of the Big Bang theory.

Theoretical work in the 1960s “showed that the universe could have had a singularity, a big bang, if the theory of relativity was correct (Stephen Hawking).” Mathematician Roger Penrose and physicist Stephen Hawking went on to prove in 1970 that there must have been a Big Bang singularity, provided only that Einstein’s theory of general relativity is correct and that the universe contains only as much matter as we observe. We will see later that this latter condition is probably not true, but for now, the Big Bang theory prevails.